Occupant detection systems that use a heater element for both seat heating and occupant detection are known. Some of these systems operate by periodically disconnecting the heater element from the power supply providing energy to heat the heater element so the occupant detection circuit can use the heater element as an occupant sensor electrode. An example of such a system is described in U.S. Pat. No. 8,091,960 issued to Kincaid et al. on Jan. 10, 2012. In order to detect if an occupant is present, and/or determine if the occupant is seated properly, it is desirable to repeat frequently the occupant detection, every ten milliseconds (10 ms), for example. However, such frequent interruptions undesirably reduce the maximum power that can be delivered to the heater element, and may undesirably generate electronic noise in the vehicle electrical system.
It has been proposed to replace the circuitry used to disconnect periodically the heater element from the power supply, as described above, with a common mode choke. An example of such a system is described in United States Patent Publication Number 2011/0121618 published by Fischer et al. on May 26, 2011. However, the desire to use a broad range of excitation signal frequencies, for example 5 kHz to 2 MHz, made providing a suitable common mode choke problematic. In particular, the desire to operate at relative low excitation signal frequencies (5 kHz to 200 kHz) in order to detect and compensate the occupant detection for high humidity and wet seat conditions necessitates an undesirably large and heavy common mode choke.